Two-optical signal generator for generating two optical signals having adjustable optical frequency difference

ABSTRACT

A two-optical signal generator is provided for generating two optical signals, where a difference between optical frequencies or optical wavelength of the two optical signals can be adjusted. A first optical modulator modulates a single-mode optical signal generated by a first light source according to an inputted signal, and outputs a modulated optical signal including predetermined specific two optical signals having a predetermined optical frequency difference, while a second light source generates a multi-mode optical signal including predetermined two further optical signals having substantially the same wavelengths as those of the predetermined specific two optical signals of the modulated optical signal, respectively. Then an optical injection device optically injects the modulated optical signal into the second light source, and the predetermined specific two optical signals of the modulated optical signal are injection-locked into the predetermined two further optical signals of the multi-mode optical signal, so that the second light source generates an injection-locked predetermined specific two optical signals.

This application is a Divisional of co-pending application Ser. No.10/724,055, filed on Dec. 1, 2003; which is a divisional of applicationSer. No. 09/511,095, filed on Feb. 23, 2000; and for which priority isclaimed under 35 U.S.C. § 120; and this application claims priority ofApplication Nos. 11-44857 and 2000-17031 filed in Japan on Feb. 23, 1999and Jan. 26, 2000 under 35 U.S.C. § 119; the entire contents of all arehereby incorporated by reference

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a two-optical signal generator for usein an optical fiber link system or the like, and in particular, totwo-optical signal generator for generating two optical signals, where adifference between optical frequencies or optical wavelengths of the twooptical signals can be adjusted. In the specification, the differencebetween the optical frequencies is referred to as an optical frequencydifference hereinafter, and the difference between the opticalwavelength is referred to as an optical wavelength differencehereinafter.

2. Description of the Related Art

An optical fiber link system modulates a digital data signal into anoptical signal, transmits a modulated optical signal to a radio basestation, performs photoelectric conversion for a received optical signalto output a radio signal, which is then power-amplified andradio-transmitted from an antenna of a radio base station.

FIG. 11 is a block diagram showing a configuration of an optical fiberlink system of a prior art.

Referring to FIG. 11, a light source 1 such as a semiconductor laser orthe like modulates an optical signal according to an inputted digitaldata signal, and outputs a modulated optical signal as a first opticalsignal (optical frequency f1) via an optical combining circuit 3 and anoptical branch circuit 4 to an optical amplifier 5. On the other hand, alight source 2 such as a semiconductor laser or the like has its opticalfrequency controlled by an optical frequency controller 10, andgenerates and outputs an optical signal as a second optical signal(optical frequency f2) via the optical combining circuit 3 and theoptical branch circuit 4 to the optical amplifier 5. In this case, adifference |f1−f2| in the optical frequency is set to, for example, aradio frequency in a millimeter-wave band of several tens to severalhundreds GHz, as shown in FIG. 13. The optical amplifier 5 amplifies apower of an inputted optical signal, and then, transmits apower-amplified optical signal to an optical receiver 200 via an opticalfiber cable 300 for connecting an optical transmitter 101 and an opticalreceiver 200 located in the radio base station.

On the other hand, a mixture optical signal obtained by mixing the firstand second optical signal branched by the optical branch circuit 4 isphotoelectrically converted by a photoelectric converter 6 whichcomprises a high-speed photodiode with a nonlinear photoelectricconversion characteristic, and then, the photoelectrically convertedsignal is frequency-converted into a high-frequency signal having afrequency lower than that of the photoelectrically converted signal by afrequency converter which consists of a millimeter wave signaloscillator 7 and a mixer 8. Then, from the components of the thusconverted high-frequency signal, a high-frequency signal, which is inproportion to an optical frequency difference |f1−f2| and which has beengenerated by the above-mentioned nonlinear photoelectric conversioncharacteristic, is taken out by a band-pass filter 9, and then, isoutputted to an optical frequency controller 10. In such an opticalfrequency loop circuit as configured above, based on the inputtedhigh-frequency signal, the optical frequency controller 10 controls theoptical frequency f2 of the second optical signal generated from thelight source 2 so that the above-mentioned optical frequency difference|f1−f21| becomes the constant. That is, an interference componentbetween the two optical signals is taken out by the photoelectricconverter 6 so that an oscillation frequency difference between theoscillation frequency of the light source 1 and that of the light source2 becomes a millimeter wave frequency, the taken interference componentis compared in frequency with the millimeter-wave frequency of themillimeter-wave signal generator 7, and then, the optical frequency ofthe light source 2 is controlled in accordance with its error signal.The optical transmitter 101 is disclosed in, for example, a first priorart document of, R. P. Braun, et al., “Optical millimeter-wavegeneration and transmission experiments for mobile 60 GHz bandCommunications,” Electronics Letters, Vol. 32, pp. 626-627, 1996(hereinafter referred to as a first prior art).

In the optical receiver 200, an optical amplifier 11 receives an opticalsignal through the optical fiber cable 300, and then, outputs the sameoptical signal to a photoelectric converter 12. The photoelectricconverter 12 comprises a high-speed photodiode having a nonlinearphotoelectric conversion characteristic, photoelectrically converts theinputted optical signal into an electric signal, and outputs the sameelectric signal to a band-pass filter 13. From the signal components ofthe photoelectrically converted signal, the band-pass filter 13 takesout a radio signal of a millimeter-wave band corresponding to theoptical frequency difference f0=|f1−f2| which has been generated by theabove-mentioned nonlinear photoelectric conversion characteristic, andthen, outputs the same radio signal to a radio transmitter 14. The radiotransmitter 14 comprises a power amplifier which power-amplify theinputted radio signal, and transmits the same radio signal via anantenna 15 toward, for example, an antenna 91 connected to a radioreceiver 210 shown in FIG. 12.

FIG. 12 is a block diagram showing a configuration of the radio receiver210 according to the first prior art.

Referring to FIG. 12, the radio signal received by the antenna 91 isamplified by a low-noise amplifier 92, which then outputs the receivedradio signal to a mixer 94 via a band-pass filter 93 which passestherethrough only a radio signal having a frequency f0 of themillimeter-wave band. The mixer 94 mixes the inputted radio signal witha local oscillation signal having a local oscillation frequency equal toan addition result of the above-mentioned millimeter-wave frequency f0generated by a millimeter-wave signal oscillator 95 to a predeterminedintermediate frequency, so as to generate a received base-band signalhaving an intermediate frequency of a frequency difference between thesetwo signals, and then, outputs the received base-band signal, via aband-pass filter 96 which passes therethrough only the signal componentof the intermediate frequency band, and via a signal amplifier 97 to ademodulator (not shown). Then, the demodulator demodulates the receivedbase-band signal into the original digital data signal.

Also, a second prior art document of, D. S. George et al., “FurtherObservations on the Optical Generation of Millimeter-wave Signals byMaster/Slave Laser Side-band Injection Locking,” MWP'97, Post-DeadlinePapers, PDP-2, 1997, discloses a constitution of a two-optical signalgenerator (hereinafter referred to as a second prior art) utilizing aheterodyne interference of two light waves in such a configurationprovided with two single-mode semiconductor lasers that an opticalsignal from a slave laser is intensity-modulated according to asine-wave signal, and the resultant higher-order mode frequency of theintensity-modulated optical signal is locked into a frequency of amaster laser.

Also, the following optical transmission system has been proposed as asystem for transmitting optical signals using three distributed feedbacksemiconductor lasers.

Further, a third prior art document of, Z. Ahmed, et al., “Low phasenoise millimeter-wave signal generation using a passively mode-lockedmonolithic DBR laser injection locked by an optical DSBSC signal,”Electronics Letters, Vol. 31, No. 15, pp. 1254, 1995, discloses atwo-optical signal generator (hereinafter referred to as a third priorart) utilizing a heterodyne interference of two light waves, in such aconfiguration that a distributed Bragg reflection-type semiconductorlaser (hereinafter referred to as a DBR laser) having a supersaturatedabsorption layer is made to oscillate in a plurality of modes, and twoside band lights generated using an intensity modulation by an externalapparatus is injection-locked into the DBR laser.

A fourth prior art document of, L. Noel et al., “Novel Technique forHigh-Capacity 60-GHz Fiber-Radio Transmission Systems,” IEEETransactions on Microwave Theory and Techniques, Vol. 45, No. 8, August1997, discloses an optical fiber link system (hereinafter referred to asa fourth prior art) for spatial transmission of a millimeter-wavesignal. The optical fiber link system comprises:

-   -   (a) a millimeter-wave light source for generating two optical        signals having a millimeter-wave band frequency difference from        each other, by using two, first and second distributed feedback        semiconductor lasers; and    -   (b) a third distributed feedback semiconductor laser having an        optical frequency different from those of these lasers of        millimeter-wave light sources, where a generated optical signal        from the third distributed feedback semiconductor laser is        directly modulated according to a data signal. In this fourth        prior art, at a transmission apparatus, two optical signals        generated by the above-mentioned millimeter-wave light source        and an optical signal generated by the above-mentioned third        distributed feedback semiconductor laser are        wavelength-multiplexed and transmitted. On the other hand, at a        receiving apparatus, the former two optical signals and the        latter optical signal are wavelength-separated by an optical        filter or the like, the respective separated optical signals are        photoelectrically converted into electric signals by a        photoelectric converter, and then, one of these        photoelectrically converted electric signals is mixed with a        predetermined local oscillation signal to obtain an original        millimeter-wave signal.

Further, a fifth prior art document of, R. P. Braun et al.,“Low-Phase-Noise Millimeter-Wave Generation at 64 GHz and DataTransmission Using Optical Sideband Injection Locking,” IEEE PhotonicsTechnology Letters, Vol. 10, No. 5, pp. 728-730, May 1998, discloses asystem (hereinafter referred to as a fifth prior art) having such aconfiguration that a digital data signal is inputted as a bias currentinto a first distributed feedback semiconductor laser so as to directlyintensity-modulate an optical signal generated by this semiconductorlaser according to the sine-wave signal, and then, higher-ordermodulation components of the resultant optically modulated signal isinjection-locked into second and third distributed feedbacksemiconductor lasers via a 3-dB photo-coupler so as to obtain a two-modeoptical signal. In this fifth prior art, when weak modulation isconducted on the second or third distributed feedback semiconductorlaser, this leads to an effect of AM-PM conversion due to an action ofinjection locking and then to such a effect that the optical frequencyof the locked output light becomes constant with a phase beingmodulated. The system of the fifth prior art utilizes this phasemodulation and transmits the phase-modulated optical signal.

However, the first prior art suffers from such a problem that the phasenoise characteristic of the millimeter-wave signal deteriorates due to alimitation of frequency stabilization by using a frequency controlcircuit, and this leads to that the optical transmitter of the firstprior art cannot be used as it is in radio communications.

Also, the second prior art suffers from such a problem that, althoughthe millimeter-wave frequency can be changed by adjusting the modulationfrequency of the sine-wave signal, the setting precision of thefrequencies fluctuate with a range up to approximately 200 MHz, andthen, it is extremely low.

Further, the third prior art has the distributed feedback optical filterin the laser, so suffers from a small frequency range in which the lasercan oscillate, and also from a high Q value as a laser resonator, andthis results in not only a small locking pull-in range, but also a smallvariable range of the carrier wave frequency.

Furthermore, in the fourth prior art, for the purpose ofwavelength-separation, it is necessary to provide expensive opticalfilters for each of the radio base stations at the receiving side, andit is also necessary to provide an additional mixer in a signalprocessing circuit for processing electric signals. This leads to thatit is necessary to provide a lot of electric parts for high frequencies.Therefore, there is such a problem that the cost of the radio basestation becomes extremely high as the number of radio base stationsincreases.

Still further, the fifth prior art has such a feature that it is notnecessary to provide any modulation for superimposing a millimeter wavesignal onto an optical signal. However, the fifth prior art suffers fromsuch a problem that it is necessary to provide three distributedfeedback semiconductor lasers which are well matched in oscillationfrequency.

SUMMARY OF THE INVENTION

A first object of the present invention is to solve the above-mentionedproblems, and to provide a two-optical signal generator, capable ofchanging the frequency of the differences between those of the twooptical signals generated by the two-optical signal generator, with arange wider than those of the first to third prior arts, and having ahigh precision for setting the frequencies.

A second object of the present invention is to solve the above-mentionedproblems, and to provide a two-optical signal generator, which has aconfiguration simpler than that of the fourth prior art, which has amanufacturing cost lower than that of the fourth prior art, and which iscapable of transmitting an optical signal according to a data signal.

A third object of the present invention is to solve the above-mentionedproblems, and provide a two-optical signal generator, which can beconstituted by comprising only two distributed feedback semiconductorlasers having different oscillation frequencies, and which is capable oftransmitting an optical signal according to a data signal.

In order to achieve the aforementioned objective, according to oneaspect of the present invention, there is provided a two-optical signalgenerator comprising:

-   -   a first light source for generating a single-mode optical        signal;    -   first optical modulation means for modulating the optical signal        generated by said first light source according to an inputted        signal, and outputting a modulated optical signal including        predetermined specific two optical signals having a        predetermined optical frequency difference;    -   a second light source for generating a multi-mode optical signal        including predetermined two further optical signals having        substantially the same wavelengths as those of the predetermined        specific two optical signals of the modulated optical signal,        respectively; and    -   optical injection means for optically injecting the modulated        optical signal outputted from said first optical modulation        means into said second light source,    -   wherein the predetermined specific two optical signals of the        modulated optical signal are injection-locked into the        predetermined two further optical signals of the multi-mode        optical signal, so that said second light source generates an        injection-locked predetermined specific two optical signals.

The above-mentioned two-optical signal generator preferably furthercomprises:

-   -   second optical modulation means, provided between said first        light source and said first optical modulation means, for        modulating the optical signal generated by said first light        source according to an inputted data signal, and outputting a        modulated further optical signal to said first optical        modulation means.

In the above-mentioned two-optical signal generator, the first lightsource generates a single-mode optical signal, modulates the generatedoptical signal according to an inputted data signal, and outputs amodulated further optical signal.

According to another aspect of the present invention, there is provideda two-optical signal generator comprising:

-   -   a first light source for generating a single-mode optical        signal, modulating the generated optical signal according to an        inputted signal, and outputting a modulated optical signal        including predetermined specific two optical signals having a        predetermined optical frequency difference;    -   a second light source for generating a multi-mode optical signal        including predetermined two further optical signals having        substantially the same wavelengths as those of the predetermined        specific two optical signals of the modulated optical signal,        respectively; and    -   optical injection means for optically injecting the modulated        optical signal outputted from said first light source into said        second light source, and optically injecting the optical signal        outputted from said second light source into said first light        source,    -   wherein the predetermined specific two optical signals of the        modulated optical signal are injection-locked into the        predetermined two further optical signals of the multi-mode        optical signal, so that said second light source generates an        injection-locked predetermined specific two optical signals.

The above-mentioned two-optical signal generator preferably furthercomprises:

-   -   optical modulation means, provided between said first light        source and said second light source, for modulating the optical        signal generated by said first light source according to an        inputted data signal, and outputting a modulated further optical        signal to said second light source.

According to a further aspect of the present invention, there isprovided a two-optical signal generator comprising:

-   -   a first light source for generating a single-mode optical        signal;    -   optical modulation means for modulating the optical signal        generated by said first light source according to an inputted        signal, and outputting a modulated optical signal including        predetermined specific two optical signals having a        predetermined optical frequency difference;    -   a second light source for generating a multi-mode optical signal        including predetermined two further optical signals having        substantially the same wavelengths as those of the predetermined        specific two optical signals of the modulated optical signal,        respectively, modulating the generated multi-mode optical signal        according to an inputted data signal, and outputting a modulated        multi-mode optical signal; and    -   optical injection means for optically injecting the modulated        optical signal outputted from said optical modulation means into        said second light source,    -   wherein the predetermined two further optical signals of the        modulated optical signal optically injected are injection-locked        into the predetermined specific two optical signals of the        multi-mode optical signal, and    -   wherein the injection locking is turned on or off in accordance        with a level of the data signal, thereby switching over whether        or not said second light source generates the predetermined        specific two optical signals.

According to a still further aspect of the present invention, there isprovided a two-optical signal generator comprising:

-   -   a first light source for generating a single-mode first optical        signal having a predetermined first wavelength;    -   optical modulation means for modulating the first optical signal        generated by said first light source according to an inputted        signal, and outputting a modulated first optical signal        including predetermined specific two optical signals having a        predetermined optical frequency difference;    -   a second light source for generating a single-mode second        optical signal having a second wavelength different from the        first wavelength, modulating the generated second optical signal        according to an inputted data signal, and outputting a modulated        second optical signal;    -   a third light source for generating a multi-mode optical signal        including a plurality of optical signals which are mode-coupled,        said multi-mode optical signal including:    -   (a) predetermined two further optical signals having        substantially the same wavelengths as those of the predetermined        specific two optical signals of the modulated first optical        signal, respectively, and    -   (b) another optical signal having substantially the same        wavelength as that of the second optical wavelength; and    -   optical injection means for optically injecting the modulated        first optical signal outputted from said optical modulation        means and the modulated second optical signal outputted from        said second light source, into said third light source,    -   wherein the predetermined specific two optical signals of the        modulated first optical signal are injection-locked into the        predetermined two further optical signals of the multi-mode        optical signal, and the modulated second optical signal are        injection-locked into another optical signal of the multi-mode        optical signal, and    -   wherein the two injection locking are turned on or off in        accordance with a level of the data signal, thereby switching        over whether or not said second light source generates the        predetermined specific two optical signals.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of the present invention willbecome clear from the following description taken in conjunction withthe preferred embodiments thereof with reference to the accompanyingdrawings throughout which like parts are designated by like referencenumerals, and in which:

FIG. 1 is a block diagram showing a configuration of an opticaltransmitter 101 a according to a first preferred embodiment of thepresent invention;

FIG. 2 is a block diagram showing a configuration of an opticaltransmitter 101 aa according to a modified preferred embodiment of thefirst preferred embodiment of the present invention;

FIG. 3 is a block diagram showing a configuration of an opticaltransmitter 101 b according to a second preferred embodiment of thepresent invention;

FIG. 4 is a block diagram showing an optical fiber link system providedwith an optical transmitter 101 c according to a third preferredembodiment of the present invention;

FIG. 5 is a block diagram showing an optical fiber link system providedwith an optical transmitter 101 d according to a fourth preferredembodiment of the present invention;

FIG. 6 is a graph showing an optical frequency characteristic of anoptical power level of an output optical signal from an opticalamplifier 5 shown in FIG. 5;

FIG. 7 is a block diagram showing a peripheral circuit of a Fabry-Pérotsemiconductor laser apparatus 29 according to a modified preferredembodiment of the present invention;

FIG. 8 is a graph showing spectra of two optical signals outputted fromthe optical transmitter 101 a shown in FIG. 1;

FIG. 9 is a graph showing a frequency spectrum of a radio signal whenthe two optical signals of FIG. 8 are photoelectrically converted;

FIG. 10 is a graph showing a gain characteristic of a radio frequency ofa radio signal before and after an injection locking which is caused inthe optical transmitter 101 a of FIG. 1;

FIG. 11 is a block diagram showing a configuration of an optical fiberlink system according to a first prior art;

FIG. 12 is a block diagram showing a configuration of a radio receiver210 according to the first prior art;

FIG. 13 is a graph showing an optical frequency spectra of two opticalsignals generated by an optical transmitter 101 of FIG. 11; and

FIG. 14 is a graph showing an electrical frequency spectrum of anelectric signal after photoelectric conversion in an optical transmitter200 of FIG. 11.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments according to the present invention will bedescribed below with reference to the attached drawings. Throughout thedrawings, the same or similar parts are indicated by the same referencenumerals.

First Preferred Embodiment

FIG. 1 is a block diagram showing a configuration of an opticaltransmitter 101 a according to the first preferred embodiment of thepresent invention. As shown in FIG. 1, the rough configuration of theoptical transmitter 101 a according to the present preferred embodimentis characterized in that an optical signal generated by a single-modedistributed feedback semiconductor laser apparatus 21 isintensity-modulated according to a high-frequency signal having apredetermined radio frequency f_(RF)/2 by using a second externaloptical modulator 25 of, for example, a Mach-Zehnder optical modulator,so as to generate a master optical signal, which is then opticallyinjected into a Fabry-Pérot semiconductor laser apparatus 29 so thatpredetermined two-mode optical signals are injection-locked into thesemiconductor laser apparatus 29, and this leads to that two opticalsignals having an optical frequency difference of the radio frequencyf_(RF) can be generated from a multi-mode optical signal.

First of all, the configuration of the optical transmitter 101 aaccording to the first preferred embodiment will be explained withreference to FIG. 1.

Referring to FIG. 1, a single-mode optical signal generated by thedistributed feedback semiconductor laser apparatus 21 is inputted as amaster optical signal to a first external optical modulator 22, whichintensity-modulates the same master optical signal according to aninputted digital data signal, and then, outputs an intensity-modulatedoptical signal to the second external modulator 25 via an opticalisolator 23 and a polarization-preserving optical fiber cable 24.

The second external optical modulator 25 is, for example, a Mach-Zehnderoptical modulator, which is formed on an optical waveguide substratemade of, for example, LiNbO₃, and which has a nonlinear opticalmodulation characteristic. ADC bias voltage for optical modulation isinputted from a DC voltage source 32 via a bias T-circuit 33 to thesecond external optical modulator 25. On the other hand, a radio signal,which becomes a radio signal of the optical fiber link system, and whichhas one half of a predetermined radio frequency f_(RF), is inputted froma reference signal generator 30 via a high-frequency amplifier 31 andthe bias T-circuit 33 to the second external optical modulator 25. Thesecond external optical modulator 25 intensity-modulates the inputtedmaster optical signal according to the inputted radio signal by usingits own nonlinear optical modulation characteristic, and then, outputsthe resulting intensity-modulated optical signal to the Fabry-Pérotsemiconductor laser apparatus 29 of a slave oscillator, via an opticalfilter 26, an optical circulator 27, and a polarization-preservingoptical fiber cable 28, namely, the intensity-modulated optical signalis optically injected into semiconductor apparatus 29. Accordingly, theoptical circulator 27 and the polarization-preserving optical fibercable 28 constitute optical injection means.

The optical signal, which is intensity-modulated by the second externalmodulator 25, includes at least the following signals:

-   -   (a) an optical carrier wave having an oscillation wavelength of        the distributed feedback semiconductor laser apparatus 21;    -   (b) side band signals of an optical signal corresponding to the        digital data signal which is used upon intensity-modulating the        optical carrier wave;    -   (c) further side band signals of predetermined specific two        optical signals having an optical frequency difference of the        above-mentioned radio frequency f_(RF).

The above-mentioned optical filter 26 is of, for example, a band-passfilter, and is provided for removing or eliminating unnecessary sidebands and carrier waves which may occur at the second external opticalmodulator 25, and for passing therethrough only predetermined specifictwo optical signals (side bands) having the optical frequency differenceof a desired radio frequency f_(RF). It is to be noted that, in the casewhere the second external optical modulator 25 of a Mach-Zehnder opticalmodulator is driven at such an operating point that gives the maximumloss, unnecessary carrier waves can be remarkably reduced, and in thiscase, it is not necessary to insert any optical filter 26.

In the present preferred embodiment, an antireflection film of adielectric multi-layered film (AR coating layer) is formed on an endsurface located at a light incident side of a semiconductor laser mediumof the Fabry-Pérot semiconductor laser apparatus 29, so that thereflectivity is reduced to about 20 to 5 or several %, and this resultsin decrease in the Q value of the Fabry-Pérot semiconductor apparatus29. In the Fabry-Pérot semiconductor laser apparatus 29, light-emittingparameters such as a temperature, an injection current or the like areadjusted so as to generate by itself such a plurality of multi-modeoptical signals, which include predetermined specific two opticalsignals having substantially the same wavelengths as those of theabove-mentioned predetermined specific two optical signals,respectively, and which are mode-coupled with each other.

In the preferred embodiment, a term “predetermined specific two opticalsignals having substantially the same wavelengths” means two opticalsignals that can be pulled-in by the injection locking. In other words,it means two optical signals located in frequency or wavelength within apull-in range of the injection locking.

Then, from these multi-mode optical signals, the Fabry-Pérotsemiconductor laser apparatus 29 selectively generates, by using theabove-mentioned injection locking, a predetermined two-mode opticalsignals having the optical frequency difference of a predetermined radiofrequency f_(RF), and outputs the two-mode optical signals through thepolarization-preserving optical fiber cable 28, the optical circulator27, and an optical power amplifier 5.

In the optical transmitter 101 a as constituted above, an optical signalgenerated by the distributed feedback semiconductor laser apparatus 21is intensity-modulated according to a high-frequency signal by thesecond external optical modulator 25 to generate a master opticalsignal, which is in turn optically injected into the Fabry-Pérotsemiconductor laser apparatus 29, so that two optical signal of themaster optical signal are injection-locked into the predeterminedspecific two optical signals of the multi-mode optical signal, and then,the predetermined specific two optical signals can be selectivelygenerated from the multi-mode optical signals. In other words, use ofthe Fabry-Pérot semiconductor laser apparatus 29 with a small Q valueleads to not only a wider pull-in range of the injection locking, butalso a wider variable range of millimeter-wave carrier wave frequencies.

Further, since the setting precision of the frequencies is determinednearly depending on the purity of frequencies of a reference sine-wavemodulation signal, then there can be obtained a stable carrier wavefrequency with relatively small phase noise after photoelectricconversion at the optical transmitter.

Still further, since the Fabry-Pérot semiconductor laser apparatus 29has a large multi-mode oscillation bandwidth, this leads to suchadvantageous effects that the distributed feedback semiconductor laserapparatus 21 of the master light source has a wider range for selectingits own oscillation frequency, and also there are such advantageouseffects as low manufacturing cost and convenient for wavelengthmultiplexing.

In other words, upon changing the oscillation frequency of the opticalsignal, it is advantageously sufficient to replace only the distributedfeedback semiconductor laser apparatus 21 with another one.

Although the above-mentioned preferred embodiment utilizes as amodulation signal, the present invention is not limited to this, and ahigh-frequency signal having one half of the desired radio frequencyf_(RF), a high-frequency signal having ¼ or ⅛ of the desired radiofrequency f_(RF) may be used as the modulation signal. In this case, thenonlinear characteristic of the second external optical modulator 25 canbe used to obtain two optical signals (side bands) having a desiredoptical frequency difference.

In the above-mentioned preferred embodiment according to the presentinvention, although a Mach-Zehnder optical modulator is used as thesecond external optical modulator 25, the present invention is notlimited to this. An optical phase modulator may be used tophase-modulate the master optical signal, so as to generate desired twooptical signals (side bands). In this case, an oscillation light of thecarrier optical signal of the distributed feedback semiconductor laserapparatus 21 remains as it is, so that this oscillation light is removedby a Fiber-Bragg grating or Fabry-Pérot resonator used as the opticalfilter 26.

The distributed feedback semiconductor laser apparatus 21 may becombined with the first external optical modulator 22, namely, thedistributed feedback semiconductor laser apparatus 21 may be provided soas to have a modulation function of the first external optical modulator22 to provide and utilize a distributed feedback semiconductor laserapparatus provided with, for example, an electric-field absorption type(EA) modulator which has been known to those skilled in the art. In thiscase, the nonlinear characteristic of the electric-field absorption typeoptical modulator is utilized.

Alternatively, the first external optical modulator 22 and the secondexternal optical modulator 25 may be exchanged in mounting position tocombine the distributed feedback semiconductor laser apparatus 21 andthe second external optical modulator 25, namely, distributed feedbacksemiconductor laser apparatus 21 may be provide with a modulationfunction of the second external optical modulator to provide and utilizea distributed feedback semiconductor laser apparatus provided with anelectric-field absorption type (EA) optical modulator which has beenknown to those skilled in the art.

FIG. 2 is a block diagram showing a configuration of an opticaltransmitter 101 aa, which is the former combination, according to amodified preferred embodiment of the first preferred embodiment of thepresent invention. In the modified preferred embodiment shown in FIG. 2,as compared with the first preferred embodiment shown in FIG. 1, thedistributed feedback semiconductor laser apparatus 21 and the firstexternal optical modulator 22 are combined so as to be integrated into adistributed feedback semiconductor laser apparatus 21 b inconfiguration. In this case, the distributed feedback semiconductorlaser apparatus 21 b has a nonlinear optical modulation characteristic,intensity-modulates an optical signal generated by itself according toan inputted digital data signal, and outputs the modulated masteroptical signal to the second external optical modulator 25 via theoptical isolator 23 and the polarization-preserving optical fiber cable24.

Second Preferred Embodiment

FIG. 3 is a block diagram showing a configuration of an opticaltransmitter 101 b according to a second preferred embodiment of thepresent invention. The rough configuration of the optical transmitter101 b of FIG. 2 is characterized in constituting a mutual-injectionlocking type optical oscillation system, in which a single-modedistributed feedback semiconductor laser apparatus 21 a is configured asa pass type one, an optical signal generated by distributed feedbacksemiconductor laser apparatus 21 a is optically injected into aFabry-Pérot semiconductor laser apparatus 29, and an optical signalgenerated by the Fabry-Pérot semiconductor laser apparatus 29 isoptically injected into the distributed feedback semiconductor laserapparatus 21 a.

Referring to FIG. 3, a high-frequency signal having one half of a radiofrequency f_(RF) is applied as a bias current from a reference signalgenerator 30 via a high-frequency amplifier 31 to the distributedfeedback semiconductor laser apparatus 21 a. The distributed feedbacksemiconductor laser apparatus 21 a has a nonlinear optical modulationcharacteristic, and frequency-modulates its own generated optical signalaccording to an inputted high-frequency signal, to generate two-modeoptical signals having an optical frequency difference of the desiredradio frequency f_(RF), and then, the generated two-mode optical signalsare optically injected into the Fabry-Pérot semiconductor laserapparatus 29, via an optical filter 40 for removing unnecessary sidebands from the same two-mode optical signals and passing therethroughthe predetermined specific two optical signals, an optical isolator 41,an external optical modulator 42 for intensity-modulating the twooptical signals according to a digital data signal, an opticalcirculator 43, and a polarization-preserving optical fiber cable 44.Then, in a manner similar to that of the first preferred embodiment,from the multi-mode optical signals generated by itself, the Fabry-Pérotsemiconductor laser apparatus 29 selects the predetermined two-modeoptical signals having the optical frequency difference of thepredetermined radio frequency f_(RF), by using the above-mentionedinjection locking, and then, outputs the selected predetermined two-modeoptical signals, via the polarization-preserving optical fiber cable 44,the optical circulator 43, an optical branch circuit 45, and an opticalpower amplifier 5. Another optical signal branched from the opticalbranch circuit 45 is fed back to another end surface of the distributedfeedback semiconductor laser apparatus 21 a. This constitutes apass-type semiconductor laser apparatus.

In other words, the distributed feedback semiconductor laser apparatus21 a, the optical filter 40, the optical isolator 41, the externaloptical modulator 42, the optical circulator 43, and the optical branchcircuit 45 are formed in a shape of loop, in which the optical signalgenerated by the distributed feedback semiconductor laser apparatus 21 ais optically injected into the Fabry-Pérot semiconductor laser apparatus29, and the optical signal generated by the Fabry-Pérot semiconductorlaser apparatus 29 is optically injected into the distributed feedbacksemiconductor laser apparatus 21 a. This constitutes a mutual injectionlocking type optical oscillation system.

The optical transmitter 101 b as configured above has the advantageouseffects similar to those of the first preferred embodiment. Since thetwo laser apparatuses 21 a and 29 mutually inject the generated opticalsignals into another one, the stability of long-term frequency precisioncan be improved even when there is caused a change in the temperature.Further, the optical transmitter 101 b of the present preferredembodiment has an advantageous effect of a simple construction since itis not necessary to provide any second external optical modulator 25shown in FIG. 1.

Third Preferred Embodiment

FIG. 4 is a block diagram showing a configuration of an optical fiberlink system provided with an optical transmitter 101 c, according to thethird preferred embodiment of the present invention. The roughconfiguration of this optical transmitter 101 c according to the thirdpreferred embodiment is characterized in that, a single-mode opticalsignal generated by a distributed feedback semiconductor laser apparatus21 are intensity-modulated by an external optical modulator 25 accordingto a radio signal having one half of a radio frequency f_(RF) theresultant intensity-modulated optical signal including two opticalsignals (side bands) having an optical frequency difference of the radiofrequency f_(RF) is optically injected into a Fabry-Pérot semiconductorlaser apparatus 29 via an optical circulator 27 and apolarization-preserving optical fiber cable 28, to injection-lockpredetermined specific two optical signals of the above-mentionedoptically-injected and intensity-modulated optical signal, intopredetermined specific two optical signals of the above-mentionedmulti-mode optical signals, so that the above-mentioned injectionlocking operation is turned on or off in accordance with a level ofdigital data signal to which a predetermined DC bias voltage is applied,and then, it is switched over whether or not the Fabry-Pérotsemiconductor laser apparatus 29 generates the above-mentionedpredetermined specific two optical signals.

It is to be noted that the configuration of the stages following theoptical circulator 27 is similar to that of the first preferredembodiment shown in FIG. 11.

Referring to FIG. 4, the external optical modulator 25 intensitymodulates a single-mode optical signal generated by the distributedfeedback semiconductor laser apparatus 21 according to a radio signalhaving one half of the radio frequency f_(RF) generated by a referencesignal generator 30, and optically injects the resultantintensity-modulated optical signal including predetermined specific twooptical signals (side bands) having an optical frequency difference ofthe radio frequency f_(RF) into the Fabry-Pérot semiconductor laserapparatus 29 via an optical isolator 23, a polarization-preservingoptical fiber cable 24, an optical filter 26, the optical circulator 27,and the polarization-preserving optical fiber cable 28. On the otherhand, the inputted digital data signal of, for example, a pulse signal,is applied to a bias T-circuit 52, and a predetermined DC bias voltagesupplied from a DC voltage source 51 is applied to the digital datasignal. The digital data signal thus biased only by the DC bias voltageis applied to the Fabry-Pérot semiconductor laser apparatus 29 as aninjection current, and then, a direct modulation is conducted.

In this case, in the Fabry-Pérot semiconductor laser apparatus 29, in amanner similar to that of the first preferred embodiment, the Q value islowered so as to generate multi-mode optical signals. Theabove-mentioned digital data signal is of, for example, a binary signalhaving high and low levels which are different from each other, and theabove-mentioned DC bias voltage is adjusted and set to either of thefollowing two cases.

(a) Case 1

When the digital data signal has the high level, the Fabry-Pérotsemiconductor laser apparatus 29 becomes such an operating or enablestate that an injection current exceeds a predetermined threshold value.In this state, the predetermined specific two optical signals of theoptical signals after intensity-modulation, which are optically injectedvia the external optical modulator 25 from the distributed feedbacksemiconductor laser apparatus 21, are injection-locked into thepredetermined specific two optical signals of the above-mentionedmulti-mode optical signals (in an ON state of injection locking), sothat the Fabry-Pérot semiconductor laser apparatus 29 generates theabove-mentioned predetermined specific two optical signals correspondingto two modes which have simultaneously become the synchronous stablestate, and then, outputs the generated predetermined specific twooptical signals to an optical receiver 200 via thepolarization-preserving optical fiber cable 28, the optical circulator27, an optical amplifier 5, and an optical fiber cable 300. On the otherhand, if the digital data signal has the low level, the Fabry-Pérotsemiconductor laser apparatus 29 becomes a disable state (in an OFFstate of injection locking) since the injection current is less than theabove-mentioned threshold value, so that the Fabry-Pérot semiconductorlaser apparatus 29 does not generate the above-mentioned predeterminedspecific two optical signals having levels above a predeterminedsignificant level.

(b) Case 2

When the digital data signal has the low level, the Fabry-Pérotsemiconductor laser apparatus 29 becomes such an operative or enablestate that the injection current exceeds the predetermined thresholdlevel. In this state, the predetermined specific two optical signals ofthe optical signals after intensity-modulation, which are opticallyinjected via the external optical modulator 25 from the distributedfeedback semiconductor laser apparatus 21, are injection-locked into thepredetermined specific two optical signals of the above-mentionedmulti-mode optical signals (in an ON state of injection locking), sothat the Fabry-Pérot semiconductor laser apparatus 29 generates theabove-mentioned predetermined specific two optical signals correspondingto two modes which have simultaneously become the synchronous stablestate, and then, outputs the generated predetermined specific twooptical signals to the optical receiver 200 via thepolarization-preserving optical fiber cable 28, the optical circulator27, the optical amplifier 5, and the optical fiber cable 300. On theother hand, if the digital data signal has the high level, theFabry-Pérot semiconductor laser apparatus 29 becomes a saturation statesince the injection current inputted into the semiconductor laserapparatus 29 becomes extremely large, and modes other than the modes ofthe above predetermined specific two optical signals becomes predominant(in an OFF state of injection locking). Accordingly, the above-mentionedpredetermined specific two optical signals having levels above thepredetermined significant level are not generated.

As described above, in both the cases 1 and 2, when turning on or offthe above-mentioned injection locking operation in accordance withswitching over between high and low levels of the digital data signal towhich a predetermined DC bias voltage is applied, it is possible toswitch over whether or not the Fabry-Pérot semiconductor laser apparatus29 generates the above-mentioned predetermined specific two opticalsignals with a predetermined significant quenching ratio. By thisswitching operation, the above-mentioned predetermined specific twooptical signals are turned on or off, that is, the radio transmitter 14turns on or off the radio signal having the millimeter frequency f_(RF),which is the optical frequency difference between the frequencies of theabove-mentioned predetermined specific two optical signals. Therefore,for example, at a radio receiver 210 shown in FIG. 12, a binary radiosignal with the radio carrier wave turned on or off is received, and asignal amplifier 97 can obtain a received binary base-band signal at itsoutput end.

Accordingly, according to the present preferred embodiment, it is notnecessary to provide any optical filter of the fourth prior art, and thedigital data signal is inputted into the Fabry-Pérot semiconductor laserapparatus 29 for direct modulation. Therefore, the optical transmitter101 c of the present preferred embodiment has a configuration simplerthan that of the fourth prior art, and has a manufacturing cost cheaperthan that of the fourth prior art. Also, the optical transmitter 101 cof the present preferred embodiment can transmit an optical signalaccording to a digital data signal.

Fourth Preferred Embodiment

FIG. 5 is a block diagram showing a configuration of an optical fiberlink system provided with an optical transmitter 101 d according to thefourth preferred embodiment of the present invention, and FIG. 6 is agraph showing optical frequency characteristics of an optical powerlevel of an output signal from an optical amplifier 5 of FIG. 5.

As shown in FIGS. 5 and 6, the optical transmitter 101 d of the fourthpreferred embodiment is characterized in that a single-mode opticalsignal having a first wavelength (optical frequency of f11) generated bya distributed feedback semiconductor laser apparatus 21 areintensity-modulated by an external optical modulator 25 according to aradio signal with one half of the radio frequency f_(RF), and then, afirst optical signal after intensity modulation including predeterminedspecific two optical signals (side bands for optical frequencies f1 andf2; where f1=f11−Δf, f2=f11+Δf) having an optical frequency differenceof the radio frequency f_(RF) is optically injected into a Fabry-Pérotsemiconductor laser apparatus 29 via an optical circulator 27 and apolarization-preserving optical fiber cable 28. On the other hand, asecond single-mode optical signal having a second wavelength (opticalfrequency f12) different from the first wavelength isintensity-modulated according to a digital data signal, and then, theintensity-modulated second optical signal is optically injected into theFabry-Pérot semiconductor laser apparatus 29 via the optical circulator27 and the polarization-preserving optical fiber cable 28. In this case,the above-mentioned predetermined specific two optical signals (havingoptical frequencies f1 and f2, respectively) of the intensity-modulatedfirst optical signal are injection-locked into the predeterminedspecific two optical signals (having optical frequencies f1 and f2,respectively) of the above-mentioned multi-mode optical signals, whileat the same time, the above-mentioned second optical signal (opticalfrequency f12) after intensity modulation are injection-locked into afurther one optical signal (optical frequency f12) of theabove-mentioned multi-mode optical signals. Then, when turning on or offboth of the above-mentioned injection locking operations in accordancewith the level of the above-mentioned data signal, the Fabry-Pérotsemiconductor laser apparatus 29 switches over whether or not theFabry-Pérot semiconductor laser apparatus 29 generates theabove-mentioned predetermined specific two optical signals.

It is to be noted that the configuration of the stages following theoptical circulator 27 is similar to that of the first prior art shown inFIG. 11.

Referring to FIG. 6, the external optical modulator 25intensity-modulates a single-mode optical signal having the firstwavelength (optical frequency f11) generated by the distributed feedbacksemiconductor laser apparatus 21 according to a radio signal with onehalf of the radio frequency f_(RF) generated by a reference signalgenerator 30, and the resultant intensity-modulated optical signalincluding the predetermined specific two optical signals (side bands ofoptical frequencies f1 and f2) having the optical frequency differenceof the radio frequency f_(RF) is optically injected into the Fabry-Pérotsemiconductor laser apparatus 29 via an optical isolator 23, alight-combining photo-coupler 63, a polarization-preserving opticalfiber cable 24, an optical filter 26, the optical circulator 27, and thepolarization-preserving optical fiber cable 28. On the other hand, theinputted digital data signal is applied as an injection current into adistributed feedback semiconductor laser apparatus 21 c, which in turngenerates the second optical signal (optical frequency f12) having thesecond wavelength, and which also intensity modulates theabove-mentioned second optical signal generated according to theinputted digital data signal, and then, the resultant secondintensity-modulated signals is optically injected into the Fabry-Pérotsemiconductor laser apparatus 29 via an optical attenuator 62, anoptical isolator 61, the photo-coupler 63, the polarization-preservingoptical fiber cable 24, the optical filter 26, the optical circulator27, and the polarization-preserving optical fiber cable 28. In thiscase, the Fabry-Pérot semiconductor laser apparatus 29 has its Q valuedecreased in a manner similar to that of the first preferred embodiment,and generates the multi-mode optical signals, which are coherent witheach other and are also mode-coupled with each other, and which includeoptical signals having various wavelengths substantially coincident withthose corresponding to the above-mentioned optical frequencies f1, f2and f12.

In the present preferred embodiment, the above-mentioned digital datasignal is of, for example, a binary signal having the high and lowlevels different from each other.

When the above-mentioned digital data signal has the low level, thesecond optical signal (optical frequency f12) generated by thedistributed feedback semiconductor laser apparatus 21 c areinjection-locked into the optical signals with the optical frequency f12of the multi-mode optical signals generated by the Fabry-Pérotsemiconductor laser apparatus 29 (in an ON state of injection locking).On the other hand, when the above-mentioned digital data signal has thehigh level, an attenuation amount of the optical attenuator 61 iscontrolled to adjust the amount of optical injection of the secondoptical signal into the Fabry-Pérot semiconductor laser apparatus 29 soas to avoid the above-mentioned injection locking (in an OFF state ofinjection locking). In this case, the level of the second optical signal(optical frequency f12) generated by the distributed feedbacksemiconductor laser apparatus 21 c changes with a predeterminedsignificant quenching ratio corresponding to the level of theabove-mentioned digital data signal, and in accordance with this, thelevel of the second optical signal (optical frequency f12) generated bythe above-mentioned injection locking changes in a similar manner.Turning on and off operation of the injection locking for the secondoptical signal causes an amplification factor of the injection lockingof the Fabry-Pérot semiconductor laser apparatus 29 to change, and thisleads to turning on or off of the injection locking of the two opticalsignals which are the side bands of the first optical signal having theoptical frequency f11 mode-coupled with the second optical signal havingthe optical frequency f12.

In other words, the saturation state of the Fabry-Pérot semiconductorlaser apparatus 29 is modulated to be turned on or off in accordancewith the level of the digital data signal, namely, in accordance withthe level of the second optical signal with the optical frequency f12.

Therefore, when the digital data signal has the low level and soinjection locking is turned on, the predetermined specific two opticalsignals of the intensity-modulated optical signal optically injectedfrom the distributed feedback semiconductor laser apparatus 21 via theexternal optical modulator 25 is injection-locked into the predeterminedspecific two optical signals of the above-mentioned multi-mode opticalsignals, the Fabry-Pérot semiconductor laser apparatus 29 generates theabove-mentioned predetermined specific two optical signals correspondingto two modes which have simultaneously become a synchronously stablestate, and then, outputs the generated predetermined specific twooptical signals to an optical receiver 200 via thepolarization-preserving optical fiber cable 28, the optical circulator27, the optical amplifier 5, and an optical fiber cable 300. On theother hand, if the digital data signal has the high level, the injectionlocking is turned off, so that the above-mentioned predeterminedspecific two optical signals having a level above a predeterminedsignificant level are not generated.

As described above, when turning on or off the above-mentioned injectionlocking in accordance with switching over between the high and lowlevels of the digital data signal, the Fabry-Pérot semiconductor laserapparatus 29 can switch over whether or not the Fabry-Pérotsemiconductor laser apparatus 29 generates the above-mentionedpredetermined specific two optical signals. By this switching operation,the above-mentioned predetermined specific two optical signals areturned on or off, namely, the radio transmitter 14 turns on or off theradio signal having the millimeter-wave frequency f_(RF), which is anoptical frequency difference between the frequencies of theabove-mentioned predetermined specific two optical signals. Therefore,for example, at the radio receiver 210 shown in FIG. 12, the binaryradio signal with its radio carrier wave being turned on or off can bereceived, and there is obtained a received base-band binary signal at anoutput end of the signal amplifier 97.

In the fifth prior art, it is necessary to provide the three distributedfeedback semiconductor laser apparatuses having matched oscillationfrequencies. On the other hand, in the present preferred embodiment, theFabry-Pérot semiconductor laser apparatus 29 has a wide range ofoscillation frequencies, and this in turn makes wide, the oscillationfrequency selection range of the distributed feedback semiconductorlaser apparatus 21 c, and then, it is not necessary to provide selectionof oscillation frequencies of the light sources. Accordingly, it ispossible to constitute the two-optical signal generator, which has asimple configuration provided with two distributed feedbacksemiconductor lasers having different oscillation frequencies, and whichcan transmit the optical signal according to the digital data signal.

Modified Preferred Embodiments

FIG. 7 is a block diagram showing a peripheral circuit of a Fabry-Pérotsemiconductor laser apparatus 29 of a modified preferred embodiment.

In the first preferred embodiment shown in FIG. 1, an optical circuitconsisting of the optical circulator 27, the polarization-preservingoptical fiber cable 28, and the Fabry-Pérot semiconductor laserapparatus 29 is inserted between the optical filter 26 and the opticalpower amplifier 5, as shown in FIG. 7. Alternatively, in order to removethe optical circulator 27, optical isolators 46 and 47 may be providedon both sides of both end surfaces of a laser medium of the Fabry-Pérotsemiconductor laser apparatus 29, and this leads to a pass type ofFabry-Pérot semiconductor laser apparatus 29 through which the opticalsignal passes.

In this case, by forming an antireflection film on both end surfaces ofthe Fabry-Pérot semiconductor laser apparatus 29, the Q value thereof isreduced.

Also in the third and fourth preferred embodiments, the Fabry-Pérotsemiconductor laser apparatus 29 of FIG. 7 may be used in a similarmanner. Further, in the second preferred embodiment shown in FIG. 3, theoptical circuit of FIG. 7 may be inserted between the external opticalmodulator 42 and the optical branch circuit 45 in a similar manner.

Although the above-mentioned preferred embodiments have the distributedfeedback semiconductor laser apparatuses 21 b and 21 c for performingintensity modulation according to a digital data signal, the presentinvention is not limited to this. There may be utilized any othermodulation such as phase modulation, frequency modulation or the likefor generating at least side bands on both sides. Also, although theexternal optical modulator 25 is provided for intensity modulation, thepresent invention is not limited to this. There may be utilized anyother modulation such as phase modulation, frequency modulation or thelike for generating at least side bands on both sides.

Although in the above-mentioned preferred embodiments the distributedfeedback semiconductor laser apparatus 21 a performs frequencymodulation for optical signals according to a digital data signal, thepresent invention is not limited to this. Any other modulation may beused such as intensity modulation, phase modulation or the like.

Although in the above-mentioned preferred embodiments there is used theFabry-Pérot semiconductor laser apparatus 29 with a reduced Q value, thepresent invention is not limited to this. There may be used any othertype of light source such as a laser apparatus which emits multi-modeoptical signals.

Although in the above-mentioned preferred embodiments the referencesignal generator 30 generates high-frequency signals having one half ofthe predetermined radio frequency f_(RF), the present invention is notlimited to this.

There may be used a signal having an appropriate frequency lower thanthose of the high-frequency signals.

Experiments and Experimental Results

The results of experiments using the optical transmitter 101 a of thefirst preferred embodiment will be described below. In the Fabry-Pérotsemiconductor laser apparatus 29, when assuming that an adjacent modeinterval is approximately 60 GHz, and one half of the sine-wave signalfrequency f_(RF) applied to the external optical modulator 25 is near 30GHz, two predetermined specific mode optical signals areinjection-locked so as to be selected and amplified from multi-modesoptical signals. In order to observe high-frequency carrier wave signalsafter photoelectric conversion by the optical receiver, the high-speedphotodiode 12 (whose band width is 50 GHz) and a spectrum analyzer wereused.

A spectrum of an outputted optical signal at the time of injectionlocking is shown in FIG. 8. The oscillation wavelength of thedistributed feedback semiconductor laser apparatus 21 was 1549.75 nm,the output optical intensity of the external optical modulator 25 was−18 dBm, one half of the modulation frequency f_(RF) was 30 GHz, theinjection current into the Fabry-Pérot semiconductor laser apparatus 29was 58.5 mA, the ambient temperature was 20.0° C., and the opticalintensity of a spectrum synchronized was −1 dBm.

A high-frequency spectrum at 60 GHz after photoelectric conversion inthis case is shown in FIG. 9. As apparent from FIG. 9, an intensity of−26.3 dBm was obtained including a conversion efficiency of thephotodiode 12. Also, with a frequency offset of 100 kHz from the maximumpeak, a good phase-noise characteristic value of −89 dBc/Hz wasobtained. Also, the frequency of the carrier wave was changed byadjusting the modulation frequency of the high-frequency signal appliedto the external optical modulator 25, and then, we checked ahigh-frequency gain (See FIG. 10) to millimeter-wave frequencies, wherethe high-frequency gain is defined as a ratio in high-frequencyintensity of an optical output power from the Fabry-Pérot semiconductorlaser apparatus 29 at the time of injection locking to an optical outputpower from the external optical modulator 25. This check came up withresults of a maximum gain of 33 dB at 60 GHz with a wide gain half-widthof 59 to 64 GHz. Also, a generation range of the carrier wave signal (orradio signal) was found to be wide in a range from 46 GHz to 70 GHz.This is considered due to a wide pull-in range of injection lockingcaused by a lower Q value as a resonator of the Fabry-Pérotsemiconductor laser apparatus 29. It was also found that nearly theequivalent results are obtained with a wide range for selecting thewavelength of the master light source even when the oscillationwavelength of the distributed feedback semiconductor laser apparatus 21of the master light source is 1540 nm and 1560 nm, since the emissionspectrum band width for the Fabry-Pérot semiconductor laser apparatus 29was 20 nm or larger.

As described above, the present experiment confirmed that such aconfiguration that uses the Fabry-Pérot semiconductor laser apparatus 29as a slave laser provides an ability to generate a millimeter-wavecarrier wave which has a reference frequency equal to one half of adesired carrier wave frequency, which has a wide range for variablefrequencies of the output power and also has a wide range for selectingthe wavelength of the master laser light source.

Further, the inventors conducted experiments by manufacturing theoptical transmitters 101 c and 101 d related to the third and fourthpreferred embodiments, respectively, and we confirmed that, whenintensity modulation is performed at the same time for predeterminedtwo-mode signals according to the digital data signal by using directmodulation (in third preferred embodiment) on the Fabry-Pérotsemiconductor laser apparatus, and injection of the intensity-modulatedlight (in fourth preferred embodiment), these predetermined two-modeoptical signals were switched over in accordance with the level ofbinary values of the digital data signal.

Advantageous Effects of the Invention

As described above in detail, the two-optical signal generator accordingto the first aspect of the present invention comprises a single-modefirst light source, and a multi-mode second light source. In thetwo-optical signal generator, the optical signal generated by the firstlight source is modulated according to an inputted signal, and themodulated optical signal including predetermined specific two opticalsignals having the predetermined optical frequency difference isoptically injected into the second light source, so as to injection-lockthe predetermined specific two optical signals into the predeterminedspecific two optical signals of the above-mentioned multi-mode opticalsignals, thereby generating the above-mentioned two injection-lockedpredetermined optical signals from the second light source.

Accordingly, by using, for example, a Fabry-Pérot type second lightsource with a lower Q value, the pull-in range for injection locking andthe variable range of millimeter-wave carrier frequency can be bothwide, and a setting precision for frequencies can be roughly determinedby the frequency purity of the reference sine-wave modulation signal,thereby obtaining the stable carrier wave frequency with low phase noiseafter photoelectric conversion at the optical receiver. Also, since, forexample, a Fabry-Pérot type second light source has a wide multi-modeoscillation band width, there is provided a wide range for selecting anoscillation frequency of the first light source on the master laser, andthis leads to advantageous effects of cost reduction and convenient uponwavelength multiplexing. That is, when changing the oscillationwavelength of the optical signal, only the first light source needs tobe replaced.

Also, the two-optical signal generator according to the second aspect ofthe present invention comprises (a) the first light source formodulating the single-mode optical signal according to an inputtedsignal and outputting the optical signal after modulation including thepredetermined specific two optical signals having the predeterminedoptical frequency difference, and (b) the multi-mode second lightsource. In the two-optical signal generator, the modulated opticalsignal from the first light source are optically injected into thesecond light source, the optical signal from the second light source isoptically injected into the first light source, and also thepredetermined specific two optical signals of the modulated opticalsignal are injection-locked into the predetermined specific two opticalsignals of the above-mentioned mufti-mode optical signals, then thisleads to that the second light source generates the above-mentionedpredetermined specific two optical signals injection-locked.

Accordingly, by using, for example, a Fabry-Pérot type second lightsource with a lower Q value, the pull-in range for injection locking canbe wide and the variable range of millimeter-wave carrier wave frequencycan also be wide and also the setting precision for frequencies can beroughly determined by the frequency purity of the reference sine-wavemodulation signal, thereby obtaining the stable carrier wave frequencywith low phase noise after photoelectric conversion at the opticalreceiver. Also, since the Fabry-Pérot type second light source has awide range of the mufti-mode oscillation band, the master first lightsource has a wide range for selecting oscillation frequencies, and thisresults in advantageous effects of cost reduction and convenient uponwavelength multiplexing. That is, when changing the oscillationwavelength of optical signals, advantageously only the first lightsource needs to be replaced. Further, because of the mutual injectionlocking between the first and second light sources, the long-termstability of the frequency precision is improved even in change in thetemperature. Furthermore, it is not necessary to provide any opticalmodulation means, and this leads to a simple configuration of thetwo-optical signal generator.

Furthermore, the two-optical signal generator according to the thirdaspect of the present invention comprises the single-mode first lightsource and the second light source which modulates its own multi-modeoptical signal according to a data signal. In the two-optical signalgenerator, the optical signals from the first light source are modulatedaccording to an inputted signal, the modulated optical signal includingthe predetermined specific two optical signals having the predeterminedoptical frequency difference are optically injected into the secondlight source, so as to injection-lock the predetermined specific twooptical signals of the above-mentioned modulated optical signal, intothe predetermined specific two optical signals of the above-mentionedmulti-mode optical signal. Then when turning on or off theabove-mentioned injection locking according to the level of theabove-mentioned data signal, the above-mentioned second light source isswitched over whether or not the second light source generates theabove-mentioned predetermined specific two optical signals.

Accordingly, in the two-optical signal generator, it is not necessary toprovide any optical filter of the fourth prior art. Since the digitaldata signal is inputted to the second light source for directmodulation, this leads to not only a simple configuration of thetwo-optical signal generator, but also an inexpensive manufacturing costlower than that of the fourth prior art. Further, the two-optical signalgenerator can transmit the optical signal according to the digital datasignal.

Still further, the two-optical signal generator according to the fourthpreferred embodiment of the present invention comprises (a) the firstlight source for generating the single-mode first optical signal, (b)the second light source for modulating its own single-mode secondoptical signal according to an inputted data signal, and (c) the thirdlight source for generating the multi-mode optical signal includingoptical signals mode-coupled with each other. In the two-optical signalgenerator, the first optical signal after modulation according to theinputted signal and the second optical signal from the second lightsource are optically injected into the third light source, so as toinjection-lock the predetermined specific two optical signals of theabove-mentioned modulated first optical signal into predeterminedspecific two optical signals of the above-mentioned multi-mode opticalsignal, and so as to injection-lock the above-mentioned modulated secondoptical signal into another optical signal of the above-mentionedmulti-mode optical signals. Then when turning on or off both of theabove two injection locking operations in accordance with the level ofthe above-mentioned data signal, the second light source is switchedover whether or not the second light source generates the abovepredetermined specific two optical signals from the second light source.

In the fifth prior art, it is necessary to provide three distributedfeedback semiconductor laser apparatuses which are matched inoscillation frequency. However, since the present invention has arelatively wide possible range for frequency oscillation of the thirdlight source in, for example, a Fabry-Pérot semiconductor laserapparatus, this leads to a wide range for selecting oscillationfrequencies of the second light source, and it is not necessary toselect the oscillation frequencies of the light sources. Therefore, itis possible to constitute the two-optical signal generator having asimple configuration which is provided with two-optical sources havingdifferent oscillation frequencies. Further, the two-optical signalgenerator can transmit the optical signal according to the digital datasignal.

Although the present invention has been fully described in connectionwith the preferred embodiments thereof with reference to theaccompanying drawings, it is to be noted that various changes andmodifications are apparent to those skilled in the art. Such changes andmodifications are to be understood as included within the scope of thepresent invention as defined by the appended claims unless they departtherefrom.

1. A two-optical signal generator comprising: a first light source forgenerating a single-mode optical signal, modulating the generatedoptical signal according to an inputted signal, and outputting amodulated optical signal including predetermined specific two opticalsignals having a predetermined optical frequency difference; a secondlight source for generating a multi-mode optical signal includingpredetermined two further optical signals having substantially the samewavelengths as those of the predetermined specific two optical signalsof the modulated optical signal, respectively; and optical injectionmeans for optically injecting the modulated optical signal outputtedfrom said first light source into said second light source, andoptically injecting the optical signal outputted from said second lightsource into said first light source, wherein the predetermined specifictwo optical signals of the modulated optical signal are injection-lockedinto the predetermined two further optical signals of the multi-modeoptical signal, so that said second light source generates aninjection-lacked predetermined specific two optical signals.
 2. Thetwo-optical signal generator as claimed in claim 1, further comprising:optical modulation means, provided between said first light source andsaid second light source, for modulating the optical signal generated bysaid first light source according to an inputted data signal, andoutputting a modulated further optical signal to said second lightsource.